The causes of arteriosclerosis are many and varied. No two persons have the same resisting power toward poisons that circulate in the blood. Some go through life exposed to all the infectious diseases without ever becoming infected, while others fall easy victims to every disease that comes, no matter how careful they may be, and it is quite the same in regard to the resistance of the arterial tissues. If the tubing is of first class quality and the individual does not place too much strain on it, he may live to the biblical three-score years and ten, and possess arteries which have undergone such slight changes that they are not palpable. Such a person is, however, the exception. On the other hand, if the tissue is of poor quality, even the ordinary wear and tear of life causes early changes in the vessels, and a person of forty may have hard arteries. We have described in a previous chapter the changes which normally occur in the arteries as age advances. An artery that is normal for a man of fifty years would be distinctly abnormal for a boy of fifteen. Two broad divisions of arteriosclerosis may be made: (1) congenital, or the result of inherited tendency; (2) acquired. =Congenital Form= When Dr. O. W. Holmes was asked how to live to the age of seventy, he replied that a man should begin to pick his ancestors one hundred years before he was born. Our parents determine the character of the tissues with which we start in life, and this determines our general resistance. We might properly speak of congenital arteriosclerosis where the affected individual had poor arterial tissue with which to begin life, for that, in a sense, is a congenital defect, and arterial tissue that is poor in quality is prone to disease.